HAO 2010 PROFILES IN SCIENCE: Dr. Alfred de Wijn

Contact

303-497-2171
dwijn@ucar.edu

Dr. Alfred de Wijn is a Postgraduate Research Scientist in the High Altitude Observatory of the National Center for Atmospheric Research. He received his PhD in Astrophysics in 2006 from Utrecht University. He began working at HAO in December 2006. His main research interest is in observational studies of the solar photosphere, chromosphere, and transition region using various ground-based and space-borne instruments.

Publication:

Propagation of waves from the photosphere to the chromosphere in and around plage

de Wijn, A. G.; McIntosh, S. De Pontieu (2010), "On the Propagation of p-Modes Into the Solar Chromosphere", ApJL, 702, 168.

(http://adsabs.harvard.edu/abs/2009ApJ...702L.168D).

Abstract:

In the traditional view of the solar atmosphere, 5-minute p-mode oscillations should not propagate upward because the acoustic cutoff is at periods of 3 minutes. However, it has long been known that p-mode oscillations propagate upward in and around magnetic flux concentrations. Several mechanisms have been proposed to explain this upward propagation of oscillations that have frequencies below the canonical values for the cut-off frequency in the photosphere, e.g., leakage along inclined field lines, or radiative losses in the photosphere. Recently, the advent of higher resolution observations and modeling have led to renewed interest in this topic, with suggestions that p-mode leakage can lead to formation of spicules. Our results show that waves with 5-minute periods are found to propagate only at the periphery of the plage, and only in the direction in which the field can be reasonably expected to expand. We concluded that the field inclination is critically important in the leakage of p-mode oscillations from the photosphere into the chromosphere.

Phase difference between Doppler shift in Fe I and Na I D1 at 3.3 mHz. Blue and red respectively indicate positive and negative phase difference, i.e., upward and downward propagation. Contours indicate the average Stokes V signal in Fe I at 2.5% intervals. Arrows in the bottom left corner of each panel indicate the shift between Na I D1 and Fe I using Na I D1 as reference. Rows are shifted in north by 3, 0, and −3 Hinode SOT/NFI pixels of 0.16", respectively from top to bottom. Columns are shifted east by 3, 0, and −3 NFI pixels, respectively from left to right
Phase difference between Doppler shift in Fe I and Na I D1 at 3.3 mHz. Blue and red respectively indicate positive and negative phase difference, i.e., upward and downward propagation. Contours indicate the average Stokes V signal in Fe I at 2.5% intervals. Arrows in the bottom left corner of each panel indicate the shift between Na I D1 and Fe I using Na I D1 as reference. Rows are shifted in north by 3, 0, and −3 Hinode SOT/NFI pixels of 0.16", respectively from top to bottom. Columns are shifted east by 3, 0, and −3 NFI pixels, respectively from left to right.

Publication:

Observations of solar scattering polarization at high spatial resolution

Snik, F.; de Wijn, A. G.; Ichimoto, K.; Fischer, C. E.; Keller, C. U.; Lites, B. W.

(http://adsabs.harvard.edu/abs/2010A%26A...519A..18S).

Abstract:

The weak, turbulent magnetic fields that are hypothesized to permeate most of the solar photosphere are difficult to observe, because the well-known Zeeman effect is virtually blind to them. The Hanle effect, acting on the scattering polarization in suitable lines, in principle can be used as a diagnostic for these fields. However, the prediction that the majority of the weak, turbulent field resides in intergranular lanes also poses significant challenges to scattering polarization observations because high spatial resolution is usually difficult to attain. Hinode-SOT offers unprecedented spatial resolution in combination with high polarimetric sensitivity. The CN band is known to have a significant scattering polarization signal, and is sensitive to the Hanle effect. We use Hinode/SOT observations in the CN band to show that the scattering polarization for granules (i.e., regions brighter than the median intensity of non-magnetic pixels) is significantly larger than for intergranules. We derive that the intergranules (i.e., the remaining non-magnetic pixels) exhibit (9.8 ± 3.0)% less scattering polarization for 0.2 < µ ≤ 0.3, although systematic effects cannot completely be excluded.

Stokes Q/I observations in CN band at the north limb from Hinode. The Q/I data is the average of all 15 acquired frames and is clipped between −0.3 and 0.3%. A clear, positive scattering polarization signal is observed at the limb. Spurious polarization signals due to residual image motion are present in the data at locations with a steep intensity gradient.
Stokes Q/I observations in CN band at the north limb from Hinode. The Q/I data is the average of all 15 acquired frames and is clipped between −0.3 and 0.3%. A clear, positive scattering polarization signal is observed at the limb. Spurious polarization signals due to residual image motion are present in the data at locations with a steep intensity gradient.